Cross-chain bridges are solutions that enable interoperability between heterogeneous blockchains. In contrast to the underlying blockchains, the bridges often provide inferior security guarantees and have been targets of hacks, causing damage in the range of 1.5 to 2 billion USD in 2022. The current state of bridge architectures is that they are ambiguous, and the relation between overarching architectures, underlying components, and exploits is shallow. We address this gap through a multivocal literature review covering 64 different bridges, including 31 exploits and four known bugs over three years (2021-2023). Our analysis identifies 13 architectural components of blockchain bridges. We link the components to eight types of vulnerabilities, also called design flaws. Furthermore, we identified prevention measures and proposed 11 impact reduction measures based on existing and possible countermeasures to address the imminent exploitation of design flaws. The results present an overview of the state of the art in bridge security, future research directions, and guidelines for designing and implementing secure cross-chain bridge architectures.

Decentralised Autonomous Organisations (DAOs) are self-governing democratic entities that operate and provide services via smart contracts on behalf of their users. Certain DAOs, particularly Decentralised Exchanges (DEXs), have recently expanded their market reach through cross-chain deployment and integration, giving rise to Cross-chain DAOs (XDAOs). These XDAOs face unique governance, deployment, and operations challenges, introducing added complexity and risks compared to traditional single-chain DAOs. While existing research has focused primarily on single-chain DAOs, we observe a shift in practice with DAOs to favour cross-chain deployments. The three largest protocols, Uniswap, Curve, and PancakeSwap, which together represent over half of the total value locked in DEXs, have been deployed across 22, 17, and 9 chains, respectively. This creates a research gap that our study addresses. In this study, we review 48 cross-chain deployments and numerous XDAO interactions, using data primarily from their respective discussion forums. Through thematic and root cause analysis, we identify eight requirements, deviations between DEXs, seven lessons learned for cross-chain deployment, and a framework to generalise the governance and deployment processes in XDAOs. As a result, we provide an improved classification of XDAO governance, highlighting limitations and future directions for cross-chain expansion. • A comprehensive study on cross-chain decentralised protocols and XDAO expansion. • Exploring and identifying diverse requirements of cross-chain deployments. • Exploring and inspecting various patterns of inconsistent enforcement. • Summary of lessons learned from the expansion of leading XDAOs. • Highlighting future research on XDAOs and cross-chain decentralised deployments.

Consensus rule changes in public permissionless blockchains are challenging. Changes can be contentious, and getting all participants to agree could be tedious. Notably, Bitcoin has seen centralisation tendencies in mining and development. However, how these tendencies influence governance processes of consensus evolution has received minimal attention. We explore how the evolution of blockchain systems and the governance of consensus intertwine from socio-technical aspects. Our study analyses the governmental structures in blockchain by looking into Bitcoin. We investigate consensus change processes through grounded theory, comprising quantitative and qualitative data from 34 consensus forks in two different blockchains, Bitcoin Core and Bitcoin Cash. We explore how decentralisation and governance unfold in practice. In contrast to existing studies, we revealed that centralisation tendencies among miners and developers have no direct control over consensus rules in a blockchain. Furthermore, centralisation tendencies do not affect decision-making for consensus evolution governance in the same way as they facilitate consensus attacks, such as 51% attacks. We also discovered that consensus governance is constrained by the technicalities of change and deployment techniques. Consequently, even though miners have the authority to make consensus changes and propose new blocks, they are restricted by deployment techniques and dependence on user adoption.

The runtime evolution of a system concerns the ability to make changes during runtime without disrupting the service. Blockchain systems need to provide continuous service and integrity. Similar challenges have been observed in centrally controlled distributed systems or mobile applications that handle runtime evolution, mainly by supporting compatible changes or running different versions concurrently. However, these solutions are not applicable in the case of blockchains, and thus, new solutions are required. This study investigates Bitcoin consensus evolution by analysing over a decade of data from Bitcoin's development channels using Strauss’ grounded theory approach and root cause analysis. The results show nine deployment features which form nine deployment techniques and ten lessons learned. Our results illustrate how different deployment techniques fit different contexts and pose different levels of consensus failure risks. Furthermore, we provide guidelines for risk minimisation during consensus rule deployment for blockchain in general and Bitcoin in particular.

Self-sovereign identity (SSI) is an emerging concept that shifts the control of identity to the person or entity to whom it belongs to without the need to rely on any centralized administrative authority. Within the SSI model, a digital identity wallet enables a user to establish relationships and interact with third parties in a secure and trusted manner. However, in order to perform various operations such as messaging and credential exchange, these usually require internet access. In some situations, this is not possible, and entities should be able to communicate independently of any external infrastructure in an offline setting. The objective of this paper is to design a proof-of-concept that would allow for secure, trustworthy, and privacy-preserving decentralized peer-to-peer communication without the need for any external networking infrastructure. For this, we investigate a particular case involving DIDComm and Bluetooth LE. We identify requirements for the architecture and propose an architectural framework that allows two entities to securely communicate. To show our concept's feasibility, we evaluate the existing technologies that could be used in the proposed architecture. Our findings indicate that this approach has the potential to enable a wide range of interesting use cases and can be integrated into existing digital identity wallet solutions to extend the capabilities of offline messaging in a secure and decentralized manner that goes beyond the current models that often rely on Internet connectivity.

Self-sovereign identity (SSI) is a new paradigm to digital identity management that is built on decentralized technologies and can exist without centralized third-parties for managing the identity data. Within the SSI model, a digital identity wallet enables a user to establish relationships and interact with third parties in a secure and trusted manner. At present, the existing SSI solutions rely on the Internet connection for carrying out the necessary operations such as messaging and credential verification. However, there are many places the Internet may not be accessible and other means for communication is needed. The objective of this paper is to design a proof-of-concept that would allow for secure, trustworthy, and decentralized peer-to-peer communication without the need for any external networking infrastructure. For this, we investigate a particular case involving DIDComm and Bluetooth Low Energy (BLE). We identify requirements for the architecture and propose an architectural framework that allows two entities to securely communicate and exchange verifiable credentials. Furthermore, we look at a specific use case, namely, how offline access control can be enabled within SSI between two mobile devices. We present and evaluate the implementation of offline access control system based on the proposed architecture. Through this research, and experimentation we can conclude that this approach has the potential to enable a wide range of interesting use cases and can be integrated into existing digital identity wallet solutions to extend the capabilities of offline messaging in a secure and decentralized manner that goes beyond the current models that rely on the Internet connectivity.

Tax-case processing is based on a classification of common taxable conditions, e.g. retirement benefits, social security benefits, income brackets. Based on those, groups of people fall under certain categorizations and should be treated equally. However, to maintain privacy, people from the same group might be handled anonymously by different case handlers. Consequently, tax-case processing for different individuals from the same group categorization may end up with different results, depending on the region in which the case is dealt with and which case manager treats the given case. This situation maintain privacy but violates fairness. On the other hand, some of the tax calculation is personal since there is no isolation between the taxable conditions data and the individual data. Often, taxable data provides personally identifiable queues. Furthermore, tax processing cannot take advantage of previous tax-case processing that has been already conducted; thus, reducing the system's overall efficiency for cases that has been already processed. To improve efficiency and transparency of the taxation process, we have investigated and explored architectural designs for privacy-preserving tax-case processing prototypes with the use of blockchain technology. In this article, we provide an overview of the available technologies, classify the architecture, and discuss a proof-of-concept implementation that can facilitate those goals. Based on the findings, a number of frameworks is considered and a proof-of-concept is made to showcase the design intent. We established, that in some cases it is possible to use blockchain technology for tax-case processing to enhance the privacy and improving the transparency at the same time.

Parallel computing has been in the spotlight with the advent of multi-core computers. The popular multithreading model does not scale very well when there are hundreds or thousands of cores, since it can only help exploit coarse-grained parallelism. There exist a lot of fine-grained parallelism to be exploited in I/O tasks and memory accesses during execution of a thread. Our Counter-Amdahl's Law tells us that it is more effective to parallelize the serial fraction of a parallel algorithm rather than the parallelized fraction in order to maximize the speedup. In this paper, we have proposed a Virtual Aggregated Processor that is aiming at speeding up execution of a thread through exploiting the fine-grained parallelism in I/O tasks and memory accesses. We have proposed and implemented two techniques, helper thread and I/O specialization, to demonstrate the potential effectiveness of the Virtual Aggregated Processor technology.

The use of modelling abstractions to map from items in the real-world to objects in the computational domain is useful both for the effective implementation of abstract problem solutions and for the management of software complexity. This paper discusses the new approach of agent-oriented software engineering (AOSE), which uses the notion of an autonomous agent as its fundamental modelling abstraction. For the AOSE approach to be fully exploited, software engineers must be able to gain leverage from an agent software architecture and framework, and there are several such frameworks now publicly available. At the present time however there is little information concerning the options that are available and what needs to be considered when choosing or developing an agent framework. We consider three different agent software architectures that are (or will be) publicly available and evaluate some of the design and architectural differences and trade-offs that are associated with them and their impact on agent-oriented software development. Our discussion examines these frameworks in the context of an example in the area of distributed information systems.

Genetic algorithms (GAs) are powerful search techniques that are used to solve difficult problems in many disciplines. Unfortunately, they can be very demanding in terms of computation load and memory. Parallel genetic algorithms (PGAs) are parallel implementations of GAs which can provide considerable gains in terms of performance and scalability. PGAs can easily be implemented on networks of heterogeneous computers or on parallel mainframes. We review the state of the art on PGAs and propose a new taxonomy also including a new form of PGA (the dynamic deme model) which was recently developed.

Dynamic Demes is a novel method for the parallelisation of evolutionary algorithms. It was derived as a combination of two other parallelisation algorithms: the master-slave distributed fitness evaluation model and the static subpopulation model. The authors present the algorithm, perform a theoretical analysis of its performance and present experimental results where Dynamic Demes was compared with other algorithms.